1. Field of the Invention
The present invention relates to an irradiation apparatus for applying a laser beam to a workpiece such as a semiconductor film to thereby perform various kinds of processing and a manufacturing method for a semiconductor device using the irradiation apparatus.
2. Description of the Related Art
In the past, a polycrystalline silicon film is used for a driving circuit device, switching device, etc. in a liquid crystal display and an organic EL (Electroluminescence) display device. Such a polycrystalline silicon film is formed by applying a laser beam to an amorphous silicon film formed by sputtering or the like to thereby anneal the amorphous silicon film. In performing such annealing by applying the laser beam, the spot shape of the laser beam is preferably made linear in order to improve the processing efficiency. Further, in forming a uniform polycrystalline silicon film (reduced in crystallinity deviation) by using a laser beam (linear beam) having such a linear spot shape, it is important to reduce the deviation in intensity distribution of the linear beam in the longitudinal direction of the spot shape, thereby reducing the nonuniformity of irradiation to the amorphous silicon film.
Under such circumstances, there has been proposed a method of applying a laser beam having an intensity uniformed in a plane perpendicular to an optical axis by interposing a lens array between a laser light source and a workpiece (amorphous silicon thin-film) (see Japanese Patent Laid-open No. Hei 10-244392 referred to as Patent Document 1, for example).
However, when a plurality of light beams divided by the lens array are superimposed on each other by a condenser lens toward the workpiece, interference fringes depending upon the spatial coherency of the laser light source are generated on a work surface of the workpiece. The generation of such interference fringes promotes the nonuniformity of irradiation and it is therefore undesirable. To solve this problem, there has been disclosed a technique such that a plurality of half-wave plates are provided downstream of the lens array toward the workpiece so as to be arranged at given intervals, thereby converting the adjacent light beams into two kinds of linearly polarized light having orthogonal polarization directions (see Japanese Patent No. 3947065 referred to as Patent Document 2 hereinafter, for example).